Heart rate is controlled by a part of the Autonomic Nervous System (ANS) known as the cardiac autonomic system (parasympathetic and sympathetic activity). Heart Rate Variability (HRV) is a measure of the beat-to-beat variability of a subject's heart rate and provides a valuable noninvasive mean for evaluating the functioning of the cardiac autonomic system. It is known that HRV measurement can be used for assessment of cardiac autonomic status, and that disease severity in heart failure can be assessed via continuous 24 hour HRV measurement.
Assessment of HRV from 24-hour Holter ECG (a portable ECG monitoring device) recordings has sometimes been of prognostic value in patients after Myocardial Infarction (MI) (“Heart rate variability assessment after acute myocardial infarction: pathophysiological and prognostic correlates.”, Singh N. et al. Circulation 1996; 93:1388-95) and in Congestive Heart Failure (CHF) patients (“Reproducibility of heart rate variability measures in patients with chronic heart failure.” Ponikowski P. et al, Clin. Sci. 1996; 91:391-8). However, this test is burdensome and does not provide quick results. According to a recent study, measures of HRV under physiologic stress (head-up-tilt) were able to differentiate between healthy control subjects and subjects with asymptomatic left ventricular dysfunction.
It is also known that the reproducibility of HRV in patients with CHF is poor (Ponikowski P. et al). As the clinical state of a patient deteriorates, although intrinsic HRV will fall, the standard measure of HRV does not reflect this fall because of the rise in ectopic beat frequency, which increases the degree of variability.
Reduced HRV during a single deep breath, or 1-2 minutes of repeated slow (0.1 Hz) breathing has been used as a measure of cardiac autonomic dysfunction for many years. It was shown to be better at differentiating between subjects with and without diabetes mellitus than the differences between horizontal and standing HRV and the Standard Deviation of Normal-Normal R-R intervals (SDNN), (“A simple bedside test of 1-minute heart rate variability during deep breathing as a prognostic index after myocardial infarction.”, Katz A. et al. Am. Heart J. 1999 Jul. 138:32-8).
US 2004/0059236 to Margulies Lyle Aaron et al., describes physiological monitoring for detection of ANS activity during sleep. This publication teaches detection of frequent brief micro arousals by a pulse oximetry and EEG methods. ANS changes are determined by analyzing changes in the slope variations of the rising edge of the pulsatile blood volume waveform.
U.S. Pat. No. 6,319,205 and U.S. Pat. No. 6,322,515 to Daniel A. Goor et al., describes non-invasive detection and monitoring of a physiological state or medical condition by monitoring changes in the peripheral arterial vasoconstriction in reaction to such state or condition. Changes related to cardiopulmonary distress and blood pressure are monitored in order to detect or monitor physiological state or medical condition. A test is carried out with a finger probe capable of applying a pressure on the finger by a pressurizing cuff. In this way blood pooling in the veins at the measuring site can be prevented during the test.
EP 1419730 to Dehchuan Sun et al., describes a non-invasive apparatus for monitoring the side effects to the ANS caused by drugs used to prevent acute or chronic side effects to the brain nerves, and for monitoring the aging of nervous system by measuring the “physiological age” of the patient based on the ANS. Artery sphygmograms, or heart potential electric wave signals are obtained using a sensor and analyzed. HRV parameters are calculated by spectral analysis methods such as Fourier Transform.
US2003163054 to Andreas Lubbertus Aloysius Johannes Dekker describes monitoring patient respiration based on a pleth signal. The pleth signal is analyzed to identify a heart rate variability parameter associated with respiration rate.
The prior art fails to provide simple and rapid (about 1 minute long) noninvasive methods and systems for analyzing the status of the cardiovascular system, and in particular of the coronary blood system.
It is therefore an object of the present invention to provide a noninvasive method and system for quickly diagnosing and monitoring the cardiovascular system, and in particular the coronary blood system and cardiac ischemia of a subject based on the response of the blood flow to stimulation.
It is another object of the present invention to provide a method and system for processing and analyzing the response of the blood flow to stimulation in order to indicate the physiological condition of a subject.
It is a further object of the present invention to provide a method and system for quickly diagnosing and monitoring the cardiovascular system of a subject based on blood flow measurements.
It is a still another object of the present invention to provide a method and system for quickly diagnosing and monitoring the status of the cardiovascular system of a subject based on a test that can be performed anywhere and which does not require attendance of professionals.
Other objects and advantages of the invention will become apparent as the description proceeds.